13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 4. Variation of r0 versus KI for parallel light. d = 1 mm, z0 = 0.1, 1 and 10 m Figure 5. Variation of stress-optical constant ct versus triaxiality coefficient k for PMMA for various values of the index of refraction n0 of the surrounding medium. k = 0 and 1 correspond to conditions of plane stress and plane strain, respectively. When the triaxiality factor is determined the corresponding value of the stress-optical constant, c, is calculated which subsequently is used for the determination of stress intensity factor. Fig. 5 presents the variation of the stress-optical constant ct for transmitted light for Plexiglas (PMMA) versus the triaxiality coefficient k from its plane stress (k = 0) to its plane strain value (k = 1) for various
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